JP4199394B2

JP4199394B2 - Control method of absorbent concentration in thiosulfate denitration method

Info

Publication number: JP4199394B2
Application number: JP31756199A
Authority: JP
Inventors: 彰小渕; 康信南野; 友英杉山
Original assignee: Mitsubishi Kakoki Kaisha Ltd
Current assignee: Mitsubishi Kakoki Kaisha Ltd
Priority date: 1999-11-09
Filing date: 1999-11-09
Publication date: 2008-12-17
Anticipated expiration: 2019-11-09
Also published as: JP2001129355A

Description

【０００１】
【発明の属する技術分野】
本発明は、窒素酸化物を含有する排ガス中の窒素酸化物をチオ硫酸塩溶液で吸収除去するチオ硫酸塩脱硝法における吸収液濃度の制御方法に関する。
【０００２】
【従来の技術】
半導体製造工場、石油化学工場、製鉄所などからは、比較的高濃度の窒素酸化物含有ガスが排出されるが、その窒素酸化物を脱硝処理する方法として、吸収液で窒素酸化物を吸収除去する湿式脱硝方法や触媒を用いて窒素酸化物を分解除去する乾式脱硝方法が用いられている。
【０００３】
前記湿式脱硝方法においては、水酸化アルカリやアンモニア等のアルカリ成分のみのアルカリ水溶液を吸収液として用いたアルカリ吸収法、過マンガン酸塩や亜塩素酸塩等の酸化剤を溶解したアルカリ水溶液を吸収液として用いた酸化吸収法、チオ硫酸塩や硫化物等の還元剤を溶解したアルカリ水溶液を吸収液として用いた還元吸収法などが知られている。
【０００４】
なお、一般的な排ガス中に含有されている窒素酸化物としては、ＮＯ、ＮＯ2及びＮＯとＮＯ2が反応して生成するＮ2Ｏ3などであるが、アルカリとの反応においては、ＮＯ＜ＮＯ2＜Ｎ2Ｏ3の順で反応速度が速くなり、従って、前記の順序で吸収除去しやすくなる。
【０００５】
前記のアルカリ吸収法は、ＮＯ2とアルカリとの直接的な反応と、排ガス中のＮＯとＮＯ2が反応して生成するＮ2Ｏ3とアルカリとの反応などで窒素酸化物を吸収除去する方法であり、Ｎ2Ｏ3の生成は、ＮＯ2とＮＯとの比が略１に近くなるほど生成量が多くなるが、ＮＯ2を多く含有する排ガスでは、ＮＯ2がアルカリと反応する割合が多くなるため、ＮＯとＮＯ2の反応が起こりにくくなり、全体としての脱硝効率が低い問題がある。
【０００６】
また、酸化吸収法では、ＮＯを酸化剤で酸化してＮＯ2とし、生成したＮＯ2とアルカリを反応させて窒素酸化物を吸収除去する方法あるが、反応しにくいＮＯを酸化して反応しやすいＮＯ2としているため、アルカリ吸収法よりも脱硝効率は高いが、ＮＯ2は前記した通り、Ｎ2Ｏ3よりも反応速度が遅いため、薬品費の高い酸化剤を用いる割には、脱硝効率の向上が不十分である問題がある。
【０００７】
更に、還元吸収法では、排ガス中のＮＯ2の一部を還元剤で還元してＮＯとし、排ガス中のＮＯ2とＮＯとの比を調整してＮ2Ｏ3を生成し、生成したＮ2Ｏ3とアルカリとの反応で窒素酸化物を吸収除去する方法であり、他の方法と比較して脱硝効率が高いが、安定した運転を維持するためには、ＮＯ2とＮＯとの比の制御操作が煩雑となる問題があった。また、一定期間の運転で吸収液は徐々に還元能力が低下するため、一定間隔又は連続で吸収液を排出する必要があるが、従来の装置においては、吸収部が１段であるため、排出液中の還元剤濃度が高く、排出液の処理に化学的処理を行う場合には、薬品費が嵩む問題が生じ、また生物学的処理を行う場合には、生物に対する負荷が大きくなる問題があった。
【０００８】
前記従来の夫々の問題に鑑みて、本願出願人は、特願平９−１２４６７１号で、吸収液としてチオ硫酸塩溶液を用いた還元吸収法において、吸収部を直列２段に構成し、２段目の吸収部にチオ硫酸塩溶液を供給して２段目の吸収液中のチオ硫酸塩濃度を高濃度とし、２段目の高濃度のチオ硫酸塩を含有する吸収液を１段目に循環して１段目の吸収液中のチオ硫酸塩濃度を低濃度で運転し、ＮＯとＮＯ2との比を調整してＮ2Ｏ3を効率的に生成させ、窒素酸化物の脱硝効率を向上させると共に、最終的に排出される１段目の吸収液のチオ硫酸塩濃度を低濃度とすることができることにより、前記従来の還元吸収法の問題を解決した方法を提案した。
【０００９】
前記特願平９−１２４６７１号で提案した発明においては、以前の還元吸収法の問題点であった、安定した運転維持のための煩雑な操作や、還元能力が低下した吸収液の処理における、化学的処理での薬品費が嵩む問題や、また、生物学的処理を行う場合における生物に対する負荷が大きくなる問題などは解決されたが、２段目にチオ硫酸塩溶液を一定量づつ連続的に供給して運転されるため、１段目及び２段目の吸収液のチオ硫酸塩濃度は成り行きの濃度となっていた。
【００１０】
また、チオ硫酸塩濃度を測定する方法としては、イオンクロマト法やヨウ素滴定法などが一般的に知られており、それらの方法でチオ硫酸塩濃度を測定していたため、サンプリングや分析に要する時間が非常に掛かり、分析された値に基づいてチオ硫酸塩溶液の供給を制御すると濃度の変動に追従できない問題があった。従って、２段目の吸収部のチオ硫酸塩濃度を一定の濃度に保持するために、チオ硫酸塩溶液を過剰に供給していた結果、薬品使用量が多くなり、薬品費が嵩む問題があった。更に、１段目の吸収部のチオ硫酸塩濃度が不明又はリアルタイムで判らないため、最終的に排出される排出液中のチオ硫酸塩濃度が一定せず、排出液の処理における化学的処理又は生物学的処理での装置の運転が煩雑となる問題があった。
【００１１】
従って本願出願人は、前記特願平９−１２４６７１号で提案した発明の問題点を解決することを課題として、特願平１１−１１０４４８号において、１段目及び／又は２段目の吸収部におけるチオ硫酸塩濃度を適正に制御でき、適正量の薬品使用量で、薬品費を低減することができる方法として、吸収部を直列２段に設け、２段目の吸収液の一部を１段目の吸収液として供給し、１段目の吸収部に酸化還元電位計を設け、酸化還元電位計で測定した電位値に基づいて２段目に供給するチオ硫酸塩溶液の供給量を制御することを特徴とするチオ硫酸塩脱硝法における吸収液濃度の制御方法を提案した。
【００１２】
【発明が解決しようとする課題】
前記特願平１１−１１０４４８号で提案した発明では、１段目及び／又は２段目の吸収部における吸収液のアルカリ濃度を制御してないため、ｐＨ値が変動し、それに伴って窒素酸化物の脱硝効率も変動する問題があった。特に、１段目の吸収部におけるｐＨ値の変動により、酸化還元電位計での電位値が不正確となり、従って、電位値に基づいて制御する２段目に供給されるチオ硫酸塩溶液の供給量や１段目に供給される２段目の吸収液の供給量が不安定となり、脱硝効率に及ぼす影響が極めて大きいことが判明した。
【００１３】
従って、本発明は、前記特願平１１−１１０４４８号で提案した発明の問題点を解決するために改良されたものであり、１段目及び２段目の吸収部における吸収液のｐＨ値を測定して、その値に基づいて１段目及び２段目にアルカリ溶液を供給し、夫々のｐＨ値を安定させることにより、１段目及び２段目の吸収部における吸収液のチオ硫酸塩濃度をより正確に所定濃度に維持し、脱硝効率をより高い効率で安定させることができ、また、適正量の薬品使用量で、薬品費を低減することができる方法を提供する目的で成されたものである。
【００１４】
【課題を解決するための手段】
前記目的を達成するための本発明の要旨は、請求項１に記載した発明においては、吸収液としてアルカリ性のチオ硫酸塩溶液を使用し、吸収部を直列２段に設け、２段目の吸収部にチオ硫酸塩溶液を供給し、２段目の吸収液の一部を１段目の吸収液として供給して窒素酸化物を含有する排ガス中の窒素酸化物を吸収除去するチオ硫酸塩脱硝法において、１段目の吸収部に酸化還元電位計及びｐＨ計、２段目の吸収部にｐＨ計を夫々設け、１段目の吸収部の酸化還元電位計で測定した電位値に基づいて１段目に供給する２段目の吸収液の供給量を制御し、１段目の吸収部のｐＨ計で測定したｐＨ値に基づいて１段目に供給するアルカリ溶液の供給量を制御し、２段目の吸収部のｐＨ計で測定したｐＨ値に基づいて２段目に供給するアルカリ溶液の供給量及びチオ硫酸塩溶液の供給量を制御することを特徴とするチオ硫酸塩脱硝法における吸収液濃度の制御方法である。
【００１５】
酸化還元電位計で１段目の吸収部の電位値を測定することにより、１段目吸収部のチオ硫酸塩濃度の変動がリアルタイムで測定でき、２段目の吸収部から１段目の吸収部に供給される吸収液の供給量が適正に制御され、また、２段目に供給するチオ硫酸塩溶液の供給量を制御することにより、２段目の吸収部のチオ硫酸塩濃度を適正な濃度に維持することが可能となった。更に、１段目の吸収部から最終的に排出される排出液中のチオ硫酸塩濃度がリアルタイムで測定できるため、排出液中のチオ硫酸塩濃度を一定の低い値に制御することができ、排水処理における化学的処理又は生物学的処理での装置の運転が容易となった。
【００１６】
また、１段目及び２段目の吸収部にｐＨ計を夫々設けて、夫々の吸収部におけるｐＨ値を測定し、ｐＨ値に基づいて１段目及び２段目に供給するアルカリ溶液の供給量を制御するため、夫々のｐＨ値を安定させることができ、従って、１段目及び２段目の吸収部における吸収液のチオ硫酸塩濃度をより正確に測定することができるため、チオ硫酸塩濃度を所定濃度に維持し、脱硝効率をより高い効率で安定させることができる。
【００１７】
なお、１段目の吸収液のｐＨ値は７〜９、好ましくは８前後であり、チオ硫酸塩濃度は０．０１〜１０ｍｇ／Ｌ、好ましくは０．１〜５ｍｇ／Ｌの低い値に制御され、２段目の吸収液のｐＨ値は９〜１３、好ましくは１２前後であり、チオ硫酸塩濃度は０．１〜２．０ｗｔ％の濃度の高い値に制御される。このような条件で運転することにより、ＮＯ2／ＮＯ比を適正な値に制御することができ、従ってＮ2Ｏ3を効率よく生成させ、脱硝効率を高く維持することができる。
【００１８】
【発明の実施の形態】
以下に本発明の実施の形態について図面に基づいて説明する。図１は本発明の一実施の形態の系統図である。
【００１９】
１は排ガス中の窒素酸化物を吸収除去する１段目の吸収部である第１吸収塔であり、内部に充填材が充填された充填層１Ａが設けられ、充填層１Ａよりも下部には、排ガスの導入管ａが接続されており、また、底部には吸収液の一定量が滞留される液溜まり部１Ｂが形成され、液溜まり部１Ｂと連通し、吸収液を抜き出して吸収液循環ポンプ８を介して充填層１Ａへ循環する吸収液循環管ｄ及び吸収液を抜き出して図示しない排水処理装置などに排出する吸収液抜き出し管ｅが接続されている。更に、頂部には１次処理された処理ガスを２段目の吸収部である第２吸収塔２へ導入する１次処理ガス排出管ｂが接続している。また、第１吸収塔１の液溜まり部１Ｂには、酸化還元電位値を測定する酸化還元電位計６及びｐＨ計５が液溜まり部１Ｂ内の吸収液に電極を浸漬して配置されている。
【００２０】
２は排ガス中の窒素酸化物を除去する２段目の吸収部である第２吸収塔であり、第１吸収塔１の構造と略同一構造である。充填材が充填された充填層２Ａが設けられ、充填層２Ａよりも下部には、第１吸収塔１で１次処理された処理ガスを導入する１次処理ガス排出管ｂが接続され、頂部には２次処理された処理ガスを大気中に排出する処理ガスの排出管ｃが接続されており、また、底部には、吸収液の一定量が滞留される液溜まり部２Ｂが形成され、液溜まり部２Ｂと連通し、吸収液を抜き出して吸収液循環ポンプ９を介して充填層２Ａへ循環する吸収液循環管ｇが接続されている。更に、吸収液循環管ｇは、２段目の吸収液の一部を１段目の吸収液として供給する分岐管ｆが途中から分岐して設けられ、第１吸収塔１の液溜まり部１Ｂに接続している。また、分岐管ｆには、１段目の吸収部の酸化還元電位計６で測定した電位値に基づいて１段目に供給する２段目の吸収液の供給量を制御する制御弁１３が設けられている。また、第２吸収塔２の液溜まり部２Ｂには、ｐＨ計７が液溜まり部２Ｂ内の吸収液に電極を浸漬して配置されている。
【００２１】
また、第２吸収塔２の液溜まり部２Ｂに新規なチオ硫酸塩溶液を供給するために、途中にチオ硫酸塩溶液供給ポンプ１２を配置した吸収液供給管ｋを介してチオ硫酸塩溶液槽４が接続され、また、第１吸収塔１及び第２吸収塔２の夫々の液溜まり部１Ｂ、２Ｂに新規なアルカリ溶液を供給するために、途中にアルカリ溶液供給ポンプ１０、１１を夫々配置したアルカリ供給管ｈ、ｊを介してアルカリ溶液槽３が接続されている。
【００２２】
なお、チオ硫酸塩としては、チオ硫酸ナトリウム、チオ硫酸アンモニウムなどを用いることができるが、反応性や薬品費などからチオ硫酸ナトリウムを用いるのが好ましく、また、アルカリとしては、水酸化ナトリウム、水酸化カリウム、炭酸ナトリウム、炭酸カリウムなどを用いることができるが、反応性や薬品費などから水酸化ナトリウムを用いるのが好ましい。更に、１段目の吸収液中のチオ硫酸塩濃度は０．０１〜１０ｍｇ／Ｌの低い値に制御され、２段目の吸収液中のチオ硫酸塩濃度は０．１〜２．０ｗｔ％の濃度の高い値に制御される。
【００２３】
前記１段目及び２段目の吸収部の吸収塔１、２は、充填塔構造であり、充填層を形成する充填材としては、合成樹脂、セラミック等で製造された円筒状、円柱状、鞍状等が用いられるが、脱硝効率やハンドリングの容易性等から合成樹脂製の円筒状充填材を用いるのが好ましく、また、前記の充填塔構造ではなく、棚段塔やスプレ−塔などの構造としてもよく、更に、吸収塔１、２の夫々に隣接して液溜まり槽を配置して、液溜まり部１Ｂ、２Ｂと連通した構造でもよく、本発明に用いられる吸収塔の構造は、それらの構造には限定されない。
【００２４】
以下、前記構成の脱硝装置により窒素酸化物を含有した排ガスから窒素酸化物を吸収除去する脱硝方法について述べる。
【００２５】
第１吸収塔１では、排ガスの導入管ａから排ガスが導入され、充填層１Ａを上向流で流通し、また、吸収液が吸収液循環ポンプ８を介して吸収液循環管ｄから充填層１Ａの上部に供給され、充填層１Ａを下向流で流通することにより、排ガスと吸収液が向流接触して排ガス中の窒素酸化物が吸収除去されると共に、チオ硫酸塩の還元作用でＮＯとＮＯ2との比が調整され、Ｎ2Ｏ3が効率的に生成される。
【００２６】
吸収液は、第１吸収塔１の底部に設けられた液溜まり部１Ｂに滞留されたのち、吸収液循環ポンプ８で吸引されて抜き出され、吸収液循環管ｄから充填層１Ａの上部に循環供給される。また、吸収液の一部は、吸収液抜き出し管ｅで第１吸収塔１外に抜き出され、図示しない排水処理装置で処理されたのち、河川等に放流されるが、吸収液の抜き出し量は、吸収液の組成により制御される。また、窒素酸化物が除去された処理ガスは、１次処理ガス排出管ｂから第２吸収塔２へ導入される。
【００２７】
第２吸収塔２では、１次処理ガス排出管ｂから１次処理された排ガスが導入され、充填層２Ａを上向流で流通し、また、吸収液が吸収液循環ポンプ９を介して吸収液循環管ｇから充填層２Ａの上部に供給され、充填層２Ａを下向流で流通することにより、排ガスと吸収液が向流接触して排ガス中の残存窒素酸化物が吸収除去される。なお、第１吸収塔１で排ガス中のＮＯとＮＯ2との比がチオ硫酸塩の還元作用で調整されており、また、第２吸収塔２でもチオ硫酸塩の還元作用で調整されて、Ｎ2Ｏ3が効率的に生成されているため、脱硝効率を高く維持することができる。窒素酸化物が除去された処理ガスは、処理ガス排出管ｃから大気中に排出される。
【００２８】
吸収液は、第２吸収塔２の底部に設けられた液溜まり部２Ｂに滞留されたのち、循環ポンプ９で吸引されて抜き出され、吸収液循環管ｇから充填層２Ａの上部に循環供給される。また、吸収液の一部は、分岐管ｆから１段目の吸収液として第１吸収塔１の液溜まり部１Ｂに供給される。
【００２９】
また、第１吸収塔１及び第２吸収塔２の夫々の吸収液のチオ硫酸塩濃度及びｐＨ値を調整するために、第２吸収塔２の液溜まり部２Ｂに、チオ硫酸塩溶液供給管ｋを介してチオ硫酸塩溶液槽４からチオ硫酸塩溶液供給ポンプ１２により新規なチオ硫酸塩溶液が供給され、また、第１吸収塔１及び第２吸収塔２の夫々の液溜まり部１Ｂ、２Ｂに、アルカリ供給管ｈ、ｊを介してアルカリ溶液槽３からアルカリ溶液供給ポンプ１０、１１により新規なアルカリ溶液が供給される。
【００３０】
前記における夫々の溶液の供給量制御は、酸化還元電位計６で第１吸収塔１における吸収液の電位値を測定し、図示しない制御装置を介してチオ硫酸塩濃度を算出して制御弁１３を制御することにより、第２吸収塔２から第１吸収塔１の液溜まり部１Ｂに供給される吸収液の供給量を適正に制御することができ、また、第２吸収塔２におけるｐＨ値をｐＨ計７で測定し、第２吸収塔２に供給するチオ硫酸塩溶液の供給量をチオ硫酸塩溶液供給ポンプ１２を制御することにより、第２吸収塔２の吸収液のチオ硫酸塩濃度を適正な濃度に維持することが可能となった。更に、第１吸収塔１から最終的に排出される排出液中のチオ硫酸塩濃度がリアルタイムで測定できるため、排出液中のチオ硫酸塩濃度を一定の低い値に制御することができ、排水処理における化学的処理又は生物学的処理での装置の運転が容易となった。
【００３１】
更に、第１吸収塔１及び第２吸収塔２の夫々の液溜まり部１Ｂ、２Ｂに配置されたｐＨ計５，７で、夫々の吸収液のｐＨ値を測定し、ｐＨ値に基づいてアルカリ溶液供給ポンプ１０，１１を制御してアルカリ溶液の供給量を制御するため、夫々のｐＨ値を安定させることができ、従って、第１吸収塔１の吸収液のチオ硫酸塩濃度をより正確に測定することができるため、チオ硫酸塩濃度を所定濃度に維持し、脱硝効率をより高い効率で安定させることができる。
【００３２】
なお、第１吸収塔１の吸収液中のチオ硫酸塩濃度は０．０１〜１０ｍｇ／Ｌの低い値に制御され、第２吸収塔２の吸収液中のチオ硫酸塩濃度は０．１〜２．０ｗｔ％の濃度の高い値に制御される。更に、排ガス中の窒素酸化物のＮＯ2／ＮＯ比に基づいて、第２吸収塔２の吸収液中のチオ硫酸塩とアルカリの濃度を制御することにより、ＮＯ2／ＮＯ比を適正な値に制御することができ、従ってＮ2Ｏ3を効率よく生成させ、脱硝効率を高く維持することができる。また、第１吸収塔１及び第２吸収塔２の吸収液のｐＨ値をｐＨ計５，７で夫々測定し、ｐＨ値に基づいて夫々に供給するアルカリ溶液の供給量を制御することにより、吸収液のｐＨ値を安定させることができ、従って、第１吸収塔１及び第２吸収塔２の吸収液のチオ硫酸塩濃度をより正確に測定することができるため、チオ硫酸塩濃度を所定濃度に維持し、脱硝効率をより高い効率で安定させることができる。
【００３３】
【実施例】
（実施例１）
前記脱硝方法により、半導体製造工程から排出される窒素酸化物を含有する排ガス中の窒素酸化物を除去処理する例について以下に詳述する。
第１吸収塔に供給するアルカリ溶液の供給量を第１吸収塔に設けたｐＨ計で制御し、また、第２吸収塔からの吸収液の供給を酸化還元電位計で制御して、第１吸収塔の吸収液のｐＨ値を８．０、電位値を１４０ｍｖに制御し、また、第２吸収塔に供給するアルカリ溶液の供給量及びチオ硫酸塩溶液の供給量を第２吸収塔に設けたｐＨ計で制御して、第２吸収塔の吸収液のｐＨ値を１２．０に制御し、窒素酸化物含有排ガスを処理した。
なお、排ガス中の窒素酸化物は、ＮＯ：３０５〜１４２ｐｐｍ、ＮＯ：９５５〜６６１ｐｐｍ、ＮＯ／ＮＯ2：３．１３〜４．６５であった。
その結果、処理ガス中の窒素酸化物はＮＯ：３７〜３２ｐｐｍ、ＮＯ2：６３〜５３ｐｐｍ、平均の脱硝率：９１．０％であり、また、チオ硫酸塩溶液の消費量は２５Ｌ／Ｈ、アルカリ溶液の消費量は５０Ｌ／Ｈであった。
【００３４】
（比較例１）
第１吸収塔に設けた酸化還元電位計により、第２吸収塔に供給するチオ硫酸塩溶液の供給量を制御し、初期の第１吸収塔の吸収液のｐＨ値を８．０、電位値を１４０ｍｖに設定し、また、第２吸収塔に設けたｐＨ計で第２吸収塔に供給するアルカリ溶液の供給量を制御して、第２吸収塔の吸収液のｐＨ値を１２．０に制御して排ガスを処理した。
なお、排ガス中の窒素酸化物は、ＮＯ：２５０〜１１０ｐｐｍ、ＮＯ2：８８０〜６７０ｐｐｍ、ＮＯ／ＮＯ2：３．５２〜６．０９であった。
その結果、第１吸収塔の吸収部のｐＨ値は７．０〜９．０の間で変動し、処理ガス中の窒素酸化物は、ＮＯ：３６〜２６ｐｐｍ、ＮＯ2：６４〜５４ｐｐｍ、平均の脱硝率：９０．６％であり、また、チオ硫酸塩溶液の消費量は２８Ｌ／Ｈ、アルカリ溶液の消費量は６５Ｌ／Ｈであった。
【００３５】
前記のとおり、本発明の吸収液濃度の制御方法では、脱硝効率をより高い効率で安定させることができ、また、チオ硫酸塩溶液及びアルカリ溶液の消費量を低減することができた。
【００３６】
【発明の効果】
本発明は、１段目及び２段目の吸収部における吸収液のチオ硫酸塩濃度をより正確に測定できるため、チオ硫酸塩濃度を所定濃度に維持し、脱硝効率をより高い効率で安定させることができ、また、チオ硫酸塩溶液及びアルカリ溶液の消費量を低減することができる。
【図面の簡単な説明】
【図１】本発明の一実施の形態の系統図
【符号の説明】
１：第１吸収塔
２：第２吸収塔
３：アルカリ溶液槽
４：チオ硫酸塩溶液槽
５、７：ｐＨ計
６：酸化還元電位計
８、９：吸収液循環ポンプ
１０、１１：アルカリ溶液供給ポンプ
１２：チオ硫酸塩溶液供給ポンプ
１３：制御弁[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for controlling the concentration of an absorbing solution in a thiosulfate denitration method in which nitrogen oxides in exhaust gas containing nitrogen oxides are absorbed and removed with a thiosulfate solution.
[0002]
[Prior art]
A relatively high concentration of nitrogen oxide-containing gas is discharged from semiconductor manufacturing plants, petrochemical plants, steelworks, etc., but as a method of denitrating the nitrogen oxides, nitrogen oxides are absorbed and removed with an absorbing solution. There are used wet denitration methods and dry denitration methods that decompose and remove nitrogen oxides using a catalyst.
[0003]
In the wet denitration method, an alkali absorption method using an alkaline aqueous solution containing only an alkali component such as alkali hydroxide or ammonia as an absorbing solution, or an alkaline aqueous solution in which an oxidizing agent such as permanganate or chlorite is dissolved is absorbed. There are known an oxidation absorption method used as a liquid, a reduction absorption method using an alkaline aqueous solution in which a reducing agent such as thiosulfate or sulfide is dissolved as an absorption liquid.
[0004]
Nitrogen oxides contained in general exhaust gas include NO, NO2, and N2O3 produced by the reaction of NO and NO2. However, in the reaction with alkali, NO <NO2 <N2O3. In this order, the reaction rate is increased, and therefore, absorption and removal are facilitated in the order.
[0005]
The alkali absorption method is a method of absorbing and removing nitrogen oxides by direct reaction of NO2 and alkali, reaction of N2O3 produced by reaction of NO and NO2 in exhaust gas and alkali, etc. N2O3 The amount of NO2 generated increases as the ratio of NO2 and NO approaches 1; however, in exhaust gas containing a large amount of NO2, the ratio of NO2 reacting with alkali increases, so the reaction between NO and NO2 occurs. There is a problem that the denitration efficiency as a whole becomes low.
[0006]
In the oxidation absorption method, there is a method in which NO is oxidized with an oxidant to form NO2, and the produced NO2 is reacted with an alkali to absorb and remove nitrogen oxides. However, NO2 which is difficult to react is oxidized and reacted easily. Therefore, the NOx removal efficiency is higher than that of the alkali absorption method, but NO2 has a slower reaction rate than N2O3 as described above. There is a problem.
[0007]
Furthermore, in the reduction absorption method, a part of NO2 in the exhaust gas is reduced with a reducing agent to NO, N2O3 is generated by adjusting the ratio of NO2 and NO in the exhaust gas, and the reaction between the generated N2O3 and alkali In this method, nitrogen oxides are absorbed and removed, and the denitration efficiency is higher than other methods. However, in order to maintain a stable operation, there is a problem that the control operation of the ratio of NO2 and NO becomes complicated. there were. In addition, since the reducing capacity of the absorbing solution gradually decreases during a certain period of operation, it is necessary to discharge the absorbing solution at regular intervals or continuously. When the concentration of the reducing agent in the liquid is high and chemical treatment is performed for the treatment of the effluent, there is a problem that chemical costs increase, and when biological treatment is performed, there is a problem that the burden on living organisms becomes large. there were.
[0008]
In view of the conventional problems, the applicant of the present application disclosed in Japanese Patent Application No. Hei 9-124671, in the reduction absorption method using a thiosulfate solution as an absorption liquid, the absorption part is configured in two stages in series. The thiosulfate solution is supplied to the absorption section of the second stage to increase the concentration of thiosulfate in the second stage absorption liquid, and the first stage of the absorption liquid containing the high concentration of thiosulfate in the second stage. The thiosulfate concentration in the first-stage absorbent is circulated to a low concentration, and the ratio of NO and NO2 is adjusted to produce N2O3 efficiently, improving the NOx removal efficiency of nitrogen oxides. At the same time, a method has been proposed in which the concentration of the thiosulfate in the first-stage absorption liquid finally discharged can be reduced, thereby solving the problems of the conventional reduction absorption method.
[0009]
In the invention proposed in the aforementioned Japanese Patent Application No. 9-124671, in the complicated operation for stable operation maintenance, which was a problem of the previous reducing absorption method, and in the treatment of the absorbing solution having reduced reducing ability, The problem of increasing chemical costs in chemical processing and the problem of increasing the burden on living organisms when performing biological processing has been solved. However, a constant amount of thiosulfate solution is continuously added in the second stage. Therefore, the thiosulfate concentration in the first-stage and second-stage absorbents was the desired concentration.
[0010]
As methods for measuring the thiosulfate concentration, ion chromatography, iodine titration, etc. are generally known, and since the thiosulfate concentration was measured by these methods, the time required for sampling and analysis However, when the supply of the thiosulfate solution is controlled based on the analyzed value, there is a problem that the fluctuation of the concentration cannot be followed. Therefore, in order to maintain the thiosulfate concentration in the second-stage absorption section at a constant concentration, as a result of excessive supply of the thiosulfate solution, there is a problem that the amount of chemical used increases and the chemical cost increases. It was. Furthermore, since the thiosulfate concentration in the first-stage absorption part is unknown or not known in real time, the thiosulfate concentration in the finally discharged effluent is not constant, and chemical treatment in the treatment of the effluent or There is a problem that the operation of the apparatus in biological treatment becomes complicated.
[0011]
Accordingly, the applicant of the present application aims to solve the problems of the invention proposed in Japanese Patent Application No. 9-124671, and in Japanese Patent Application No. 11-110448, the first stage and / or the second stage absorption section. As a method that can appropriately control the thiosulfate concentration in the water and reduce the chemical cost with an appropriate amount of chemical used, an absorption part is provided in two stages in series, and a part of the second stage absorbent is 1 Provided as a first-stage absorption liquid, provided with a redox potentiometer in the first-stage absorption section, and controlled the supply amount of the thiosulfate solution supplied in the second stage based on the potential value measured with the redox potentiometer A method for controlling the concentration of the absorbent in thiosulfate denitration was proposed.
[0012]
[Problems to be solved by the invention]
In the invention proposed in the Japanese Patent Application No. 11-110448, the alkali concentration of the absorbing solution in the first and / or second-stage absorption part is not controlled, so that the pH value fluctuates, and nitrogen oxidation occurs accordingly. There was a problem that the denitration efficiency of the objects also fluctuated. In particular, the fluctuation of the pH value in the first-stage absorption part makes the potential value in the oxidation-reduction potentiometer inaccurate, and therefore the supply of the thiosulfate solution supplied in the second stage that is controlled based on the potential value. It was found that the amount and the supply amount of the second-stage absorption liquid supplied to the first stage became unstable, and the influence on the denitration efficiency was extremely large.
[0013]
Therefore, the present invention has been improved in order to solve the problems of the invention proposed in Japanese Patent Application No. 11-110448, and the pH value of the absorbing solution in the first and second absorption parts is set. Based on the measured value, an alkaline solution is supplied to the first stage and the second stage, and the respective pH values are stabilized to stabilize the thiosulfate of the absorbing solution in the first and second stage absorption parts. It was made for the purpose of providing a method that can maintain the concentration more accurately at a predetermined concentration, stabilize the denitration efficiency at a higher efficiency, and reduce the chemical cost with an appropriate amount of chemical used. It is a thing.
[0014]
[Means for Solving the Problems]
The gist of the present invention for achieving the above object is that, in the invention described in claim 1, an alkaline thiosulfate solution is used as the absorbing solution, and the absorption section is provided in two stages in series. Thiosulfate denitration that supplies thiosulfate solution to the part and supplies part of the second-stage absorption liquid as the first-stage absorption liquid to absorb and remove nitrogen oxides in exhaust gas containing nitrogen oxides In the method, an oxidation-reduction potentiometer and a pH meter are provided in the first-stage absorption part, and a pH meter is provided in the second-stage absorption part, respectively, based on the potential value measured by the oxidation-reduction potentiometer of the first-stage absorption part. The supply amount of the second-stage absorption liquid supplied to the first stage is controlled, and the supply amount of the alkaline solution supplied to the first stage is controlled based on the pH value measured by the pH meter of the absorption section of the first stage. Alkaline solution supplied to the second stage based on the pH value measured by the pH meter of the second stage absorption section A method of controlling the absorption fluid concentrations of thiosulfate denitration method characterized by controlling the supply amount of the supply amount and thiosulfate solution.
[0015]
By measuring the potential value of the first-stage absorption section with an oxidation-reduction potentiometer, fluctuations in the thiosulfate concentration in the first-stage absorption section can be measured in real time, and the first-stage absorption section is absorbed from the second-stage absorption section. The supply amount of the absorbent supplied to the section is appropriately controlled, and the thiosulfate concentration in the second stage absorption section is controlled appropriately by controlling the supply amount of the thiosulfate solution supplied to the second stage. It became possible to maintain a high concentration. Furthermore, since the thiosulfate concentration in the effluent finally discharged from the first stage absorption part can be measured in real time, the thiosulfate concentration in the effluent can be controlled to a certain low value, Operation of the apparatus in chemical treatment or biological treatment in wastewater treatment became easy.
[0016]
In addition, a pH meter is provided in each of the first and second stage absorption units, the pH values in the respective absorption units are measured, and supply of an alkaline solution to be supplied to the first and second stages based on the pH value In order to control the amount, each pH value can be stabilized, and therefore the concentration of thiosulfate in the absorbing solution in the first and second stage absorption sections can be measured more accurately. The salt concentration can be maintained at a predetermined concentration, and the denitration efficiency can be stabilized with higher efficiency.
[0017]
The pH value of the first stage absorbent is 7-9, preferably around 8, and the thiosulfate concentration is controlled to a low value of 0.01-10 mg / L, preferably 0.1-5 mg / L. The pH value of the second stage absorbent is 9 to 13, preferably around 12, and the thiosulfate concentration is controlled to a high value of 0.1 to 2.0 wt%. By operating under such conditions, the NO 2 / NO ratio can be controlled to an appropriate value, so that N 2 O 3 can be efficiently generated and the denitration efficiency can be kept high.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a system diagram of an embodiment of the present invention.
[0019]
Reference numeral 1 denotes a first absorption tower which is a first-stage absorption section that absorbs and removes nitrogen oxides in exhaust gas. A packed bed 1A filled with a filler is provided inside, and below the packed bed 1A, An exhaust gas introduction pipe a is connected to the bottom, and a liquid reservoir 1B in which a certain amount of absorption liquid is retained is formed at the bottom, communicated with the liquid reservoir 1B, and the absorption liquid is extracted to circulate the absorption liquid. An absorption liquid circulation pipe d that circulates to the packed bed 1A via the pump 8 and an absorption liquid extraction pipe e that extracts the absorption liquid and discharges it to a wastewater treatment apparatus (not shown) are connected. Further, a primary processing gas discharge pipe b for introducing the processing gas subjected to the primary processing into the second absorption tower 2 as the second stage absorption section is connected to the top. Further, in the liquid reservoir 1B of the first absorption tower 1, an oxidation-reduction potentiometer 6 and a pH meter 5 for measuring the oxidation-reduction potential value are arranged with electrodes immersed in the absorption liquid in the liquid reservoir 1B. .
[0020]
Reference numeral 2 denotes a second absorption tower which is a second stage absorption section for removing nitrogen oxides in the exhaust gas, and has substantially the same structure as that of the first absorption tower 1. A packed bed 2A filled with a filler is provided, and a lower part of the packed bed 2A is connected to a primary processing gas discharge pipe b for introducing a processing gas primary-treated in the first absorption tower 1, Is connected to a processing gas discharge pipe c for discharging the processing gas subjected to the secondary processing to the atmosphere, and a liquid reservoir 2B in which a certain amount of the absorbing liquid is retained is formed at the bottom, An absorbing liquid circulation pipe g that communicates with the liquid reservoir 2B and extracts the absorbing liquid and circulates it to the packed bed 2A via the absorbing liquid circulation pump 9 is connected. Further, the absorption liquid circulation pipe g is provided with a branch pipe f for supplying a part of the second-stage absorption liquid as the first-stage absorption liquid, which is branched from the middle, and the liquid reservoir 1B of the first absorption tower 1 is provided. Connected to. The branch pipe f has a control valve 13 for controlling the supply amount of the second-stage absorption liquid supplied to the first stage based on the potential value measured by the oxidation-reduction potentiometer 6 of the first-stage absorption section. Is provided. Further, in the liquid reservoir 2B of the second absorption tower 2, a pH meter 7 is disposed by immersing an electrode in the absorption liquid in the liquid reservoir 2B.
[0021]
Moreover, in order to supply a novel thiosulfate solution to the liquid reservoir 2B of the second absorption tower 2, a thiosulfate solution tank is provided via an absorption liquid supply pipe k in which a thiosulfate solution supply pump 12 is disposed in the middle. 4 are connected, and alkali solution supply pumps 10 and 11 are arranged in the middle to supply new alkaline solutions to the liquid reservoirs 1B and 2B of the first absorption tower 1 and the second absorption tower 2, respectively. The alkaline solution tank 3 is connected through the alkali supply pipes h and j.
[0022]
As the thiosulfate, sodium thiosulfate, ammonium thiosulfate and the like can be used, but sodium thiosulfate is preferably used from the viewpoint of reactivity and chemical cost, and the alkali includes sodium hydroxide, hydroxide. Although potassium, sodium carbonate, potassium carbonate, etc. can be used, it is preferable to use sodium hydroxide from the viewpoint of reactivity and chemical cost. Further, the thiosulfate concentration in the first-stage absorption liquid is controlled to a low value of 0.01 to 10 mg / L, and the thiosulfate concentration in the second-stage absorption liquid is 0.1 to 2.0 wt%. The concentration is controlled to a high value.
[0023]
The absorption towers 1 and 2 of the first-stage and second-stage absorption sections have a packed tower structure, and as a filler forming the packed bed, a cylindrical shape, a columnar shape made of synthetic resin, ceramic, or the like, Although a bowl-like shape is used, it is preferable to use a cylindrical packing material made of a synthetic resin from the viewpoint of denitration efficiency, ease of handling, etc., and not a packed tower structure as described above, but a plate tower, a spray tower, etc. The structure may be a structure, and a liquid storage tank may be disposed adjacent to each of the absorption towers 1 and 2 to communicate with the liquid storage parts 1B and 2B. The structure of the absorption tower used in the present invention is as follows: It is not limited to those structures.
[0024]
Hereinafter, a denitration method for absorbing and removing nitrogen oxides from exhaust gas containing nitrogen oxides by the denitration apparatus having the above configuration will be described.
[0025]
In the first absorption tower 1, the exhaust gas is introduced from the exhaust gas introduction pipe a and flows upward through the packed bed 1 </ b> A, and the absorbing liquid is passed from the absorbing liquid circulation pipe d through the absorbing liquid circulation pump 8 to the packed bed. By being supplied to the upper part of 1A and flowing through the packed bed 1A in a downward flow, the exhaust gas and the absorption liquid come into countercurrent contact, and nitrogen oxides in the exhaust gas are absorbed and removed, and thiosulfate is reduced. The ratio between NO and NO2 is adjusted, and N2O3 is efficiently generated.
[0026]
The absorption liquid is retained in a liquid reservoir 1B provided at the bottom of the first absorption tower 1 and then sucked and extracted by the absorption liquid circulation pump 8 from the absorption liquid circulation pipe d to the upper part of the packed bed 1A. Circulated. Further, a part of the absorption liquid is extracted out of the first absorption tower 1 by the absorption liquid extraction pipe e, processed by a wastewater treatment device (not shown), and then discharged into a river or the like. Is controlled by the composition of the absorbent. The processing gas from which nitrogen oxides have been removed is introduced into the second absorption tower 2 from the primary processing gas discharge pipe b.
[0027]
In the second absorption tower 2, the exhaust gas subjected to the primary treatment is introduced from the primary treatment gas discharge pipe b, flows in the packed bed 2 </ b> A in an upward flow, and the absorption liquid is absorbed through the absorption liquid circulation pump 9. By being supplied from the liquid circulation pipe g to the upper part of the packed bed 2A and flowing through the packed bed 2A in a downward flow, the exhaust gas and the absorption liquid come into countercurrent contact, and the residual nitrogen oxides in the exhaust gas are absorbed and removed. In the first absorption tower 1, the ratio of NO to NO2 in the exhaust gas is adjusted by the reduction action of thiosulfate, and also in the second absorption tower 2 is adjusted by the reduction action of thiosulfate, and N2O3. Is efficiently generated, so that the denitration efficiency can be maintained high. The processing gas from which nitrogen oxides have been removed is discharged into the atmosphere from the processing gas discharge pipe c.
[0028]
The absorption liquid is retained in the liquid reservoir 2B provided at the bottom of the second absorption tower 2, and then sucked and extracted by the circulation pump 9, and circulated and supplied from the absorption liquid circulation pipe g to the upper part of the packed bed 2A. Is done. Further, a part of the absorption liquid is supplied from the branch pipe f to the liquid reservoir 1B of the first absorption tower 1 as the first-stage absorption liquid.
[0029]
Further, in order to adjust the thiosulfate concentration and pH value of the respective absorption liquids of the first absorption tower 1 and the second absorption tower 2, a thiosulfate solution supply pipe is provided in the liquid reservoir 2B of the second absorption tower 2. a new thiosulfate solution is supplied from the thiosulfate solution tank 4 by the thiosulfate solution supply pump 12 via k, and the liquid reservoirs 1B of the first absorption tower 1 and the second absorption tower 2, respectively. A novel alkaline solution is supplied to 2B from the alkaline solution tank 3 by the alkaline solution supply pumps 10 and 11 through the alkaline supply pipes h and j.
[0030]
The control of the supply amount of each solution described above is performed by measuring the potential value of the absorbing solution in the first absorption tower 1 with the oxidation-reduction potentiometer 6 and calculating the thiosulfate concentration via a control device (not shown). The amount of absorption liquid supplied from the second absorption tower 2 to the liquid reservoir 1B of the first absorption tower 1 can be controlled appropriately, and the pH value in the second absorption tower 2 can be controlled. Is measured with a pH meter 7, and the thiosulfate solution supply pump 12 controls the supply amount of the thiosulfate solution supplied to the second absorption tower 2, whereby the thiosulfate concentration in the absorption liquid of the second absorption tower 2 Can be maintained at an appropriate concentration. Furthermore, since the thiosulfate concentration in the effluent finally discharged from the first absorption tower 1 can be measured in real time, the thiosulfate concentration in the effluent can be controlled to a constant low value, Operation of the apparatus for chemical or biological treatment in the treatment is facilitated.
[0031]
Furthermore, the pH value of each absorption liquid is measured with the pH meters 5 and 7 arranged in the liquid reservoirs 1B and 2B of the first absorption tower 1 and the second absorption tower 2, respectively. Since the supply amount of the alkaline solution is controlled by controlling the solution supply pumps 10 and 11, each pH value can be stabilized. Therefore, the thiosulfate concentration of the absorption liquid in the first absorption tower 1 can be more accurately determined. Since it can be measured, the thiosulfate concentration can be maintained at a predetermined concentration, and the denitration efficiency can be stabilized with higher efficiency.
[0032]
The thiosulfate concentration in the absorption liquid of the first absorption tower 1 is controlled to a low value of 0.01 to 10 mg / L, and the thiosulfate concentration in the absorption liquid of the second absorption tower 2 is 0.1 to 10 mg / L. The concentration is controlled to a high value of 2.0 wt%. Furthermore, the NO2 / NO ratio is controlled to an appropriate value by controlling the concentration of thiosulfate and alkali in the absorption liquid of the second absorption tower 2 based on the NO2 / NO ratio of nitrogen oxides in the exhaust gas. Therefore, N2O3 can be efficiently generated, and the denitration efficiency can be kept high. Moreover, by measuring the pH value of the absorption liquid of the 1st absorption tower 1 and the 2nd absorption tower 2 with the pH meters 5 and 7, respectively, and controlling the supply amount of the alkaline solution supplied to each based on the pH value, The pH value of the absorption liquid can be stabilized, and therefore the thiosulfate concentration of the absorption liquid in the first absorption tower 1 and the second absorption tower 2 can be measured more accurately. The concentration can be maintained, and the denitration efficiency can be stabilized at a higher efficiency.
[0033]
【Example】
(Example 1)
An example in which nitrogen oxides in exhaust gas containing nitrogen oxides discharged from the semiconductor manufacturing process are removed by the denitration method will be described in detail below.
The supply amount of the alkaline solution supplied to the first absorption tower is controlled by a pH meter provided in the first absorption tower, and the supply of the absorption liquid from the second absorption tower is controlled by an oxidation-reduction potentiometer. The pH value of the absorption liquid in the absorption tower is controlled to 8.0, the potential value is controlled to 140 mV, and the supply amount of the alkaline solution and the supply amount of the thiosulfate solution supplied to the second absorption tower are provided in the second absorption tower. The pH value of the absorption liquid in the second absorption tower was controlled to 12.0, and the nitrogen oxide-containing exhaust gas was treated.
The nitrogen oxides in the exhaust gas were NO: 305 to 142 ppm, NO: 955 to 661 ppm, and NO / NO2: 3.13 to 4.65.
As a result, the nitrogen oxides in the treatment gas were NO: 37-32 ppm, NO2: 63-53 ppm, the average denitration rate: 91.0%, and the consumption of the thiosulfate solution was 25 L / H, alkaline The consumption of the solution was 50 L / H.
[0034]
(Comparative Example 1)
The supply amount of the thiosulfate solution supplied to the second absorption tower is controlled by the oxidation-reduction potentiometer provided in the first absorption tower, the pH value of the absorption liquid in the initial first absorption tower is 8.0, and the potential value Is set to 140 mV, and the pH value of the absorbing solution in the second absorption tower is set to 12.0 by controlling the supply amount of the alkaline solution supplied to the second absorption tower with a pH meter provided in the second absorption tower. The exhaust gas was treated in a controlled manner.
The nitrogen oxides in the exhaust gas were NO: 250 to 110 ppm, NO2: 880 to 670 ppm, NO / NO2: 3.52 to 6.09.
As a result, the pH value of the absorption part of the first absorption tower fluctuates between 7.0 and 9.0, and the nitrogen oxides in the process gas are NO: 36-26 ppm, NO2: 64-54 ppm, average Denitration rate: 90.6%, consumption of thiosulfate solution was 28 L / H, and consumption of alkaline solution was 65 L / H.
[0035]
As described above, in the method for controlling the concentration of the absorbing solution of the present invention, the denitration efficiency can be stabilized with higher efficiency, and the consumption of the thiosulfate solution and the alkaline solution can be reduced.
[0036]
【The invention's effect】
In the present invention, since the thiosulfate concentration of the absorbing solution in the first and second absorption sections can be measured more accurately, the thiosulfate concentration is maintained at a predetermined concentration, and the denitration efficiency is stabilized at a higher efficiency. And the consumption of thiosulfate solution and alkaline solution can be reduced.
[Brief description of the drawings]
FIG. 1 is a system diagram of an embodiment of the present invention.
1: first absorption tower 2: second absorption tower 3: alkaline solution tank 4: thiosulfate solution tank 5, 7: pH meter 6: oxidation-reduction potentiometer 8, 9: absorption liquid circulation pump 10, 11: alkaline solution Supply pump 12: Thiosulfate solution supply pump 13: Control valve

Claims

吸収液としてアルカリ性のチオ硫酸塩溶液を使用し、吸収部を直列２段に設け、２段目の吸収部にチオ硫酸塩溶液を供給し、２段目の吸収液の一部を１段目の吸収液として供給して窒素酸化物を含有する排ガス中の窒素酸化物を吸収除去するチオ硫酸塩脱硝法において、１段目の吸収部に酸化還元電位計及びｐＨ計、２段目の吸収部にｐＨ計を夫々設け、１段目の吸収部の酸化還元電位計で測定した電位値に基づいて１段目に供給する２段目の吸収液の供給量を制御し、１段目の吸収部のｐＨ計で測定したｐＨ値に基づいて１段目に供給するアルカリ溶液の供給量を制御し、２段目の吸収部のｐＨ計で測定したｐＨ値に基づいて２段目に供給するアルカリ溶液の供給量及びチオ硫酸塩溶液の供給量を制御することを特徴とするチオ硫酸塩脱硝法における吸収液濃度の制御方法。An alkaline thiosulfate solution is used as the absorption liquid, the absorption section is provided in two stages in series, the thiosulfate solution is supplied to the second absorption section, and a part of the second absorption liquid is used in the first stage. In the thiosulfate denitration method in which nitrogen oxides in exhaust gas containing nitrogen oxides are supplied and absorbed as an absorption liquid, redox potentiometer and pH meter in the first stage absorption part, second stage absorption A pH meter is provided in each section, and the supply amount of the second-stage absorbent supplied to the first stage is controlled based on the potential value measured by the oxidation-reduction potentiometer in the first-stage absorption section. The supply amount of the alkaline solution supplied to the first stage is controlled based on the pH value measured by the pH meter of the absorption part, and the second stage is supplied based on the pH value measured by the pH meter of the second stage absorption part. Thiosulfate denitration characterized by controlling the supply amount of alkaline solution and the supply amount of thiosulfate solution The method of absorbing fluid concentrations of.